CAST explores the dark side of the Universe

Following the search for axions, candidates for dark matter, CAST is widening its scientific horizon by searching for chameleons, hypothetical particles postulated as an explanation for dark energy.

CAST, CERN's axion solar telescope, moves on its rail to follow the Sun (for an hour and a half at dawn and an hour and a half at dusk).

As the summer comes to an end, surveyors have set to work in the experimental hall of CAST, CERN’s axion solar telescope. They will spend around 10 days perfecting the alignment of the detector with respect to the position of the Sun, to within a thousandth of a radian. The Sun's course is visible from the one window in the CAST experimental hall just twice a year, in March and September. This is why the physicists are making the most of these few days to align their magnet precisely.

For 12 years, CAST has been tracking the movement of the Sun for an hour and a half at dawn and an hour and a half at dusk. The experiment is searching for solar axions, hypothetical particles that are thought to interact very weakly with ordinary matter.

A time lapse video of the CAST detector's movements (Video: Madalin Rosu/CAST Collaboration).

Axions were postulated in 1977 to solve a problem related to charge-parity symmetry violation (see box). Axions, if they exist, could also be good candidates for the Universe’s dark matter, one of the great mysteries of contemporary physics. Physicists think that the Sun could also produce axions, which is why CAST is pointed towards it to capture them; however, visible light is needed only to adjust the alignment of the detector.

The CAST experimental apparatus is based on a magnet from the LHC, which has been converted into a telescope. Under the effect of its intense magnetic field, the axions (if they exist) turn into photons in the X-ray range. The X-ray detectors at each end of the magnet allow photons to be detected at sunrise and sunset. An excess of X-rays compared with the background could indicate the presence of axions.

This astroparticle experiment, which was the first of its kind at CERN, has not (yet) detected solar axions. But, as its detectors have been developed, it has established the most restrictive limit on the axion-photon coupling constant, which defines the probability that an axion will change into a photon (or vice versa) in a magnetic field. “CAST has become the global reference in the search for axions,” says Konstantin Zioutas, the experiment's spokesperson. “But we have reached the limit of the solar axion research that we can carry out with our apparatus.”

Antje Behrens, from the CERN Surveying group, Giovanni Cantatore, Deputy spokesperson of CAST, and Marin Karuza, member of the CAST collaboration, align the detector.

CAST will therefore complete its hunt for solar axions at the end of 2015. “However, we have already turned our experiment towards a new field of research – dark energy,” explains Konstantin Zioutas. “It is a tradition at CERN to launch future research programmes far in advance in order to take the necessary time to prepare the new detectors and get new collaborators interested.”

The collaboration intends to track another type of hypothetical particle – chameleons, which are candidates for dark energy (see box). Dark energy is thought to represent around 70% of the Universe's energy and to be behind the acceleration of the expansion observed in the cosmos. Over the last 10 years, theories have been developed to shed light on the nature of dark energy, involving new particles such as chameleons.

If chameleons exist, they could, like axions, be converted into X-rays under the effect of a powerful magnetic field. The theory predicts that chameleons may be produced by the Sun. However, the energy of the X-rays produced by these solar chameleons is thought to be almost 10 times weaker than that of X-rays produced by solar axions. Over the last two years, the collaboration has therefore installed new X-ray detectors with a lower energy threshold at the end of its magnet. The first is a Silicon Drift Detector, and the second is a gas detector called InGRID (Integrated Grid). Incorporating silicon-based Micromegas technology, the InGRID detector performs very well also at low energy levels.

CAST is also preparing an additional method of detecting chameleons, based on the pressure that their flux is thought to exert on a solid surface. The collaboration is putting the finishing touches to a new opto-mechanical sensor that uses an ultra-thin membrane just 100 nanometres thick. “It will be capable of detecting a displacement of around 10-15 metres, that is to say the size of the nucleus of an atom,” says Giovanni Cantatore, deputy spokesperson of CAST. “This is a sensitivity comparable to that of interferometric antennas that detect gravitational waves.”

The displacement of the membrane is detected using optical interferometry (a Fabry-Pérot interferometer). A prototype of this low-cost detector is already in operation at the INFN in Trieste. Known as KWISP (Kinetic Weakly Interacting Slim Particles detection), it is due to be installed on the CAST magnet at the beginning of next year.

CAST has submitted its full scientific programme to the SPSC Committee, which will meet in October.

To learn more about the InGRID X-ray detector, click here. To learn more about the KWISP detector, click here.

Axions wash whiter than white

Charge-parity symmetry violation, which could explain matter-antimatter asymmetry, has been observed only in processes linked to the weak interaction. Quantum chromodynamics (QCD), the theory of the strong interaction, also predicts the existence of this violation. But, until now, CP violation with the strong interaction has not been observed. Hence the development of a theory that resolves this problem by predicting the existence of as-yet-undetected particles – axions. Physicist Frank Wilczek named axions after a brand of washing powder, because their existence would allow the theory to be “cleaned up”.

A Universe full of chameleons?

According to the chameleons model, a scalar field may be the source of the dark energy that causes the Universe to expand more rapidly. The scalar fields called “chameleons” are thought to interact like a fifth force and according to the density of matter encountered. If the density is low, the force would manifest itself at long-range and explain the acceleration of the expansion of the Universe. If the density is high, the interaction range would be so small that it would be practically impossible to measure, like on Earth. With a spin of 0, chameleon particles would be manifestations of this scalar field, in a similar way to how the Higgs boson proves the existence of the Brout-Englert-Higgs scalar field.